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COMPARATIVE AND EVOLUTIONARY PHYSIOLOGY

Calcium flux in turtle ventricular myocytes

¹Faculty of Life Sciences, The University of Manchester, Core Technology Facility, Manchester, United Kingdom; and ²School of Biosciences, The University of Birmingham, Birmingham, United Kingdom

Submitted 15 June 2006 ; accepted in final form 2 August 2006

The relative contribution of the sarcoplasmic reticulum (SR), the L-type Ca²⁺ channel and the Na⁺/Ca²⁺ exchanger (NCX) were assessed in turtle ventricular myocytes using epifluorescent microscopy and electrophysiology. Confocal microscopy images of turtle myocytes revealed spindle-shaped cells, which lacked T-tubules and had a large surface area-to-volume ratio. Myocytes loaded with the fluorescent Ca²⁺-sensitive dye Fura-2 elicited Ca²⁺ transients, which were insensitive to ryanodine and thapsigargin, indicating the SR plays a small role in the regulation of contraction and relaxation in the turtle ventricle. Sarcolemmal Ca²⁺ currents were measured using the perforated-patch voltage-clamp technique. Depolarizing voltage steps to 0 mV elicited an inward current that could be blocked by nifedipine, indicating the presence of Ca²⁺ currents originating from L-type Ca²⁺ channels (I_Ca). The density of I_Ca was 3.2 ± 0.5 pA/pF, which led to an overall total Ca²⁺ influx of 64.1 ± 9.3 µM/l. NCX activity was measured as the Ni⁺-sensitive current at two concentrations of intracellular Na⁺ (7 and 14 mM). Total Ca²⁺ influx through the NCX during depolarizing voltage steps to 0 mV was 58.5 ± 7.7 µmol/l and 26.7 ± 3.2 µmol/l at 14 and 7 mM intracellular Na⁺, respectively. In the absence of the SR and L-type Ca²⁺ channels, the NCX is able to support myocyte contraction independently. Our results indicate turtle ventricular myocytes are primed for sarcolemmal Ca²⁺ transport, and most of the Ca²⁺ used for contraction originates from the L-type Ca²⁺ channel.

reptile; excitation-contraction coupling; sarcoplasmic reticulum; Na⁺/Ca²⁺ exchanger; L-type Ca²⁺ channel

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